The present invention relates to an apparatus for controlling air-fuel ratio of an internal combustion engine (engine), and a method for controlling air-fuel ratio, and specifically relates to an apparatus for controlling air-fuel ratio of an internal combustion engine, and a method for controlling air-fuel ratio, wherein fuel control is carried out by means of feedback control of air-fuel ratio in an internal combustion engine equipped with a three-way catalytic converter.
As conventional technology of an apparatus for controlling air-fuel ratio of an internal combustion engine, such one is well-known that is composed of carrying out main feedback control, which features a rapid convergence to target air-fuel ratio, using a air-fuel ratio sensor installed at the upstream side of a catalytic converter, and carrying out sub-feedback control, using an oxygen sensor installed at the downstream side of a catalytic converter, in consideration of controllability of air-fuel ratio at the transitional period of operation of an engine (for example, JP-A-2005-48711).
The main feedback control is carried out in such a way that fuel injection amount is set “rich” when air-fuel ratio of exhaust gas flowing in the catalytic converter is “leaner” than target air-fuel ratio, while fuel injection amount is set “lean” when air-fuel ratio of exhaust gas is “richer” than target air-fuel ratio.
The sub-feedback control is carried out in such a way that fuel injection amount is set “rich” when exhaust gas flowing out of the catalytic converter is “leaner” than the stoichiometric air-fuel ratio, while fuel injection amount is set “lean” when air-fuel ratio of exhaust gas flowing out of the catalytic converter is “richer” than the stoichiometric air-fuel ratio.